1. Field of the Invention
The present invention relates to synthetic multi-component fibers, especially synthetic bi-component fibers used in the manufacture of non-woven fabrics. In particular, the present invention relates to processes and apparatus for the production of multi-component polymer fibers and filaments at high speed and in a densely packed arrangement. More specifically, the present invention relates to multi-component fibers produced at high speed using one or more high hole surface density spinnerettes with subsequent high velocity quenching of the fibers.
2. Background Information
The production of multi-component polymer fibers typically involves the use of at least two different polymers which are routed in the molten state, via a complex spin pack, to the top hole of a spinnerette so that the desired cross-sectional configuration can be obtained for the resultant multi-component fibers which are extruded from the base of the spinnerette.
Multi-component fibers can be formed in many configurations, and the term "multi-component fibers" is used here to broadly include "bi-component fibers", where bi-component fibers include two different and separate polymeric components and multi-component fibers may have two or more different and separate polymeric components. Among the various bi-component fiber configurations are: the concentric sheath-core type, where a core is made of a first polymer and a concentric sheath made from a second polymer is disposed concentrically about the core; a side-by-side type, where two polymeric components are disposed side by side in parallel relationship in the fiber; and a tri-lobed configuration, where three tips of a tri-lobal shaped fiber are formed from a polymer which is different from a polymer that makes up the remainder of the fiber.
There are generally two types of processes used for producing multi-component fibers of the type referred to above. One process is the older two-step "long-spin" process which involves first melt-extruding fibers at typical spinning speeds of 500 to 3000 meters per minute, and more usually depending on the polymer to be spun from 500 to 1500 meters per minute, bundling the obtained unstretched fibers and temporarily storing them, and thereafter collecting them to form a thick tow which is fed through an apparatus, in a second step, usually run at 100 to 250 meters per minute, where the fibers are drawn, crimped, and cut into staple fiber.
The second process is a one-step "short spin" process which involves conversion from polymers to staple fibers in a single step where typical spinning speeds are in the range of 50 up to 200 meters per minute. The productivity of the one-step process is increased with the use of a much higher number of holes per spinnerette compared to that typically used in the long spin process.
Since the "short spin" process is carried out without any interruption between the spinning step and the drawing step, it is more advantageous than the "long spin" process in that higher yields can be achieved without the need for storage space for the fiber between steps, or the extra installation space needed for the "long spin" apparatus layout.
The principles of the production of molten multi-component filaments are known and are described in U.S. Pat. No. 4,738,607 to NAKAJIMA et al., which is hereby incorporated by reference in its entirety. In this patent, at least two different thermoplastic polymers are independently melted by heating to prepare independent spinning liquids, and the two liquids are separately fed under pressure to spinning holes by way of independent paths at which time, or just before which time, they are combined with each other at a predetermined ratio. The combined polymers are then extruded from the bottom holes of the spinnerette in the form of multiple multi-component fibers which must then be quenched to solidify the same.
Apparatus and methods are also known for melt spinning of polymers to obtain certain advantages in the spinning of bi-component fibers. For example, U.S. Pat. No. 4,406,850 to HILLS (HILLS '850), which is hereby incorporated by reference in its entirety, is directed to apparatus and methods for delivering a supply of different polymers to each spinning orifice in a spinnerette, while retaining a relatively high surface density of filaments per unit area of spinnerette face or surface.
HILLS '850 discloses that the most difficult type of bi-component spinning to achieve a high number of holes per unit area of spinnerette surface or high hole surface density, is the concentric sheath-core type. HILLS '850 discloses an improved spin pack design to achieve "high hole surface density" when spinning concentric sheath-core fibers. The spinnerette plate is disclosed to achieve a hole surface density of 2.0 to 2.5 passages per square centimeter of spinnerette bottom surface, and HILLS '850 states that even closer spacing is possible.
U.S. Pat. No. 5,162,074 to HILLS (HILLS '074), which is hereby incorporated by reference in its entirety, is directed to apparatus and methods for spinning multi-component fibers at an even higher hole surface density. HILLS '074 discloses a hole surface density of about eight or so spinning orifices in each square centimeter of spinnerette face area, and the positioning of the spinning orifices in staggered rows to promote more efficient fiber quenching. The HILLS '074 patent utilizes one or more disposable distributor plates in which distributor flow paths are etched on one or both sides to distribute different polymer components to appropriate spinnerette inlet hole locations.
In attempting to maximize productivity (i.e., grams of polymer per minute per square centimeter of spinnerette surface area) and fiber uniformity (i.e., denier and shape) while keeping costs as low as possible, HILLS '074, in several test runs, uses a spinnerette having spinning orifices (i.e., holes) arranged six millimeters apart in a direction perpendicular to the quench air flow, to produce a resulting hole surface density of 7.9 holes per square centimeter of spinnerette face area (i.e., bottom surface), or 12.6 square millimeters per hole. With this density, a strong quench air flow within the first 150 millimeters below the spinnerette was required to prevent marrying of the filaments. HILLS '074 does not specify the characteristics of the quench unit used, but makes use of a readily available and well known quench unit.
With all multi-component fiber manufacture via melt spinning there has been a problem with sufficiently quenching molten fibers which are spun at hole surface densities greater than one hole per 12.6 square millimeters of spinnerette lower surface. Standard quench units are incapable of sufficiently cooling molten multi-component filaments, and this results in "married" filaments wherein two or more filaments fuse together before they become sufficiently solidified. Another problem which results from insufficient cooling is "slubbing" wherein the molten filaments (i.e., fibers) are not cooled rapidly enough to withstand the spinning stress, which results in broken fibers or filaments.